Missouri is examining "Google's collection, use, and disclosure of information about Google users and their online activities; Google's alleged misappropriation of online content from the websites of its competitors; and Google's alleged manipulation of search results to preference websites owned by Google and to demote websites that compete with Google."

"There is strong reason to believe that Google has not been acting with the best interest of Missourians in mind," Hawley said. "My office will not stand by and let private consumer information be jeopardized by industry giants, especially to pad their profits."

Google said that it hasn't yet received the subpoena. "However, we have strong privacy protections in place for our users and continue to operate in a highly competitive and dynamic environment," Google spokesman Patrick Lenihan said.

The inquiry by Missouri isn't the first such probe Google has faced. The European Union in June slapped Google with a record $2.7 billion fine for favoring its own shopping services in its search results over those of rivals. The EU also has been investigating whether Google hurt app rivals by demanding its own service be preinstalled on all Android phones, as well as looking into accusations that Google blocked rivals in online search advertising. The inquiry comes also as Google, Facebook and Twitter face questions over their role in enabling the spread of fake news and the Russian misinformation campaign.